Consider the extreme example of a person exerting all efforts to defame people around them, whether in work or their own social group, collecting innocent emails and letters from people and later trying to use them as an evidence that those people have lied and are corrupt or 'unworthy'.

However, they go to all lengths to protect their own integrity and 'uprightness' by comparing themselves to the so-called liars and unworthy people by always mentioning their own good points etc.

I thought about 'Character assassin' or 'defamer' but it doesn't seem to give the full meaning.

Is their not a word / term used by psychiatrists to describe such a person?

I don't think insecure conveys the malicious nature illustrated in the question. sociopath is much more applicable. Especially the first 4 bullet points in particular.
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Jarrod RobersonMay 2 '12 at 18:06

I agree that "sociopathic" may come closest to an accurate psychological label for the behavior described, but I think that a metaphorical characterization may be even more apt. In that spirit, I recommend "sanctimonious viper." According to Merriam-Webster's Eleventh Collegiate Dictionary (2003), "sanctimonious" means "hypocritically pious or devout," which captures the aspect of false virtuousness, and a "viper" is a venomous snake.

A calumniator or slanderer is defined as someone who maliciously and without reason imputes a crime or fault to another, of which he is innocent. To calumniatemeans to make maliciously or knowingly false statements about someone.

What you describe is known as "building yourself up by tearing others down." People who engage in that behavior are probably suffering from low self-esteem, but I don't know if there is a psychiatric term that applies to such a person (or if you really should label someone with a psychiatric term).